1. Field of the Invention
The present invention relates to the field of material aging research and management, specifically to the in-situ monitoring and estimation of the condition of various degradable components used in a wide variety of applications (including, inter alia, electrical cable, process system valves, aircraft, spacecraft, and automobiles) via neutron activation techniques.
2. Description of Related Technology
The aging of degradable components (particularly those constructed in whole or in part of organic compounds such as polymers) is of great importance to modern society. Such degradable components comprise a significant fraction of what may be termed as "critical" components in use in many industrial, aerospace, and automotive applications, both commercial and military. Included in this category are components such as electrical cable insulation, valve internals, bushings, seals, and gaskets. Degradation and ultimate failure of these so-called critical components is of paramount importance in that such failures may result in the unanticipated maintenance costs, loss of operational capability and availability, and even loss of human life.
Several different approaches to managing the aging of such components exist. One approach involves 1) subjecting laboratory or in-situ specimens of a given component to a progressive regimen of aging stressors such as heat, radiation, electrical potential, chemicals, and/or oxygen present in the anticipated operating environment (known generally as "artificial aging"); 2) identifying a critical parameter of the component's function in the desired application (such as dielectric strength for an insulator); 3) determining a maximum or minimum acceptable value for the chosen parameter; 4) correlating the maximum or minimum acceptable value to a given installed lifetime (for example, via aging models such as the Arrhenius equation); and 5) removing the component from service when the installed lifetime is reached. Note, however, that this approach has the distinct disadvantage of not directly monitoring the condition of a given component, thereby introducing potentially significant variations in component condition across various applications. Specifically, some applications may have aged more or less than expected (due to a variety of factors such as radiant heat or radiation shielding, variations in oxygen/inert gas concentration, aging prior to installation, inaccuracies in the aging model used, etc.), and hence are being replaced either prematurely or too late. More effective condition monitoring programs will utilize a similar approach as that outlined above, yet instead of rotely replacing a component at a given point in life, will monitor the degradation of the component as a function of time to determine it's rate of aging as compared to the artificially (or naturally) aged specimen. The primary drawbacks of these latter condition monitoring programs include the costs of monitoring, component inaccessibility, and component/device downtime. For example, the condition monitoring of a fluoropolymer valve seat requires either remote inspection or disassembly of the valve, thereby removing the valve from service for a period of time. In such cases, simple periodic replacement of the component during other scheduled maintenance may be more cost effective. In some instances (such as electrical cable, described further below), no periodic maintenance or replacement is ever scheduled; hence condition monitoring of some sort is almost a necessity. The enormity of cost associated with replacement of cable in, for example, a commercial nuclear power facility, underscores the need for effective aging assessment and monitoring techniques.
Electrical Cable
As previously indicated, the aging and unanticipated failure of power, control, instrumentation, and data transmission cable may have significant adverse effects on plant operation and maintenance (O&M) costs and downtime. Electrical and optical cables have traditionally been considered long-lived components which merit little in the way of preventive maintenance or condition monitoring due to their generally high level of reliability and simplicity of construction. Like all other components, however, such cables age as the result of operational and environmental stressors. Aging effects may be spatially generalized (i.e., affecting most or all portions of a given cable equally, such as for a cable located completely within a single room of uniform temperature), or localized (i.e., affecting only very limited portions of a cable, such as in the case of a cable routed near a highly localized heat source). The severity of these aging effects depends on several factors including the severity of the stressor, the materials of construction and design of the cable, and the ambient environment surrounding the cable. Detailed discussions of electrical cable aging may be found in a number of publications including SAND96-0344 "Aging Management Guideline for Commercial Nuclear Power Plants--Electrical Cable and Terminations" prepared by Sandia National Laboratories/U.S. Department of Energy, September 1996. Discussions regarding optical cable aging may be found, inter alia, in Electric Power Research Institute (EPRI) publications and telecommunications industry literature. The following description will be limited to electrical cable, although it can be appreciated that the principles of aging and analysis described herein may also be largely applicable to optical cabling as well as many other types of polymeric components.
Electrical cables come in a wide variety of voltage ranges and configurations, depending on their anticipated uses. Existing prior art low- and medium-voltage power and control cables such as that shown in FIGS. 1a-1d are typically constructed using a polymer or rubber dielectric insulation 200 which is applied over a multi-strand copper or aluminum conductor 202. The insulation is often overlaid with a protective polymer jacket 204. In multi-conductor cables (such as those used in three-phase alternating current systems, as shown in FIGS. 1a and 1b), a plurality of these individually insulated conductors are encased within a protective outer jacket 206 along with other components such as filler 208 and drain wires (not shown). These other components fulfill a variety of functions including imparting mechanical stability and rigidity to the cable, shielding against electromagnetic interference, and allowing for the dissipation of accumulated electrostatic charge. This general arrangement is used for its relatively low cost, ease of handling and installation, comparatively small physical dimensions, and protection against environmental stressors.
Current methods of evaluating electrical cable component aging generally may be categorized as electrical, physical, and microphysical. Electrical techniques involve the measurement of one or more electrical parameters relating to the operation of the cable, such as the breakdown voltage, power factor, capacitance, or electrical resistance of the dielectric. These methods have to the present been considered largely ineffective or impractical, in that they either do not show a good correlation between the parameter being measured and the aging of the dielectric, or are difficult to implement under normal operations. Furthermore, such techniques are often deleterious to the longevity of the insulation, and have difficulty determining localized aging within a given conductor.
Physical techniques including the measurement of compressive modulus, torsional modulus, or rigidity under bending often show a better correlation between the aging of the cable and the measured parameter (especially for low-voltage cable), and are more practical to apply during operational conditions. However, they generally suffer from a lack of access to the most critical elements of the cable, the individual electrical conductors and their insulation. For example, the measurement of compressive modulus by way of instruments such as the Indenter Polymer Aging Monitor are effective primarily with respect to the outer, accessible surface of the cable such as its outer jacket. Although correlations of the aging of the outerjacket to that of the underlying conductors have been attempted, these correlations are generally quite imprecise and are subject to a large degree of variability based on the specific configuration of the cable being tested (i.e., its materials of construction, insulation/jacket thickness, etc.), the presence of ohmically induced heating, shielding of the conductors against stressors by the outer jacket, and differences in the oxygen concentration at the conductor insulation versus that at the outer jacket. See EPRI TR-104075, "Evaluation of Cable Polymer Aging Through Indenter Testing of In-Plant and Laboratory Aged Specimens," prepared by the Electric Power Research Institute, January, 1996 for a discussion of the correlation between outer jacket and conductor physical measurements.
Other physical techniques such as the measurement of the tensile strength or elongation-at-break of the insulation material are inherently destructive and require a specimen of the aged cable for testing.
Another potential drawback to many of the physical techniques described above is disturbance of the bulk cable run during testing. In some applications, the dielectric of the cable being evaluated may be highly aged and embrittled, yet still completely functional. However, substantial movement of the cable (such as picking the cable up and clamping on a test device) may produce localized elongation stresses beyond those corresponding to the elongation-at-break for the insulation and/or jacket material, thereby inducing unwanted cracking of the insulation and/orjacketing and potential electrical failure.
Microphysical techniques such as the measurement of insulation oxidation induction time (OIT), density, gel or plasticizer content, infrared absorption spectroscopy UV spectroscopy, and NMR are generally quite accurate, yet require samples of the cable insulation and/or jacket for analysis. For jacketed conductors, such samples are generally only available at the ends of the cable where the conductors are terminated to a source or load, and not anywhere between. Furthermore, as with the physical techniques described above, the results of any such testing are necessarily applicable only to the localized area of the cable from which the specimen was taken, which may or may not be representative of the rest of the cable. Hence, one can either take a small sample of material from the outer jacket of the cable and attempt to extrapolate the results of the aging analysis to the underlying conductors, or alternatively take a sample at the ends of the conductor itself near its terminations and extrapolate these results to the rest of the unexposed conductor. Under either alternative, a substantial degree of uncertainty and imprecision exists. Plant operators are also generally reticent to allowing the removal of even small samples of material from their cables, especially in applications where plant safety and continuity of electrical power are critical.
Another common problem in applying either physical or microphysical techniques to a localized portion of cable is the existence of conduit. In the typical power or industrial plant, many miles of cable may be encased within metallic or plastic conduit, thereby rendering it all but inaccessible. While it is true that such conduit also affords the cable additional protection from most stressors (such as heat and radiation), it also may preclude any effective estimation of aging using existing techniques. For example, the aging of a portion of nuclear plant safety-related cable contained in a conduit running directly over a large radiant heat source may be for all intents and purposes immeasurable during it's installed lifetime. While the remainder of the cable not in direct proximity to the heat source may be largely unaffected, the insulation of the cable in the region directly adjacent to the heat source may undergo dramatically accelerated aging and ultimately failure well in advance of the rest of the cable.
Fast Neutron Activation
The technique of fast neutron activation (FNA) is well known in the nuclear arts. Generally speaking, this technique employs a stream of energetic (fast) neutrons to induce secondary gamma ray emission from a target object via inelastic scattering with nuclei in the target. The gamma ray spectrum associated with a given element is unique and identifiable given sufficient energy resolution. Heretofore, FNA systems have been used exclusively in the detection and identification analysis of organic materials in obstructed locations (such as in contraband detection or bore hole exploration; see for example U.S. Pat. No. 5,098,640, "Apparatus and Method for Detecting Contraband using Fast Neutron Activation"). Such techniques, however, have not been applied to the in-situ analysis of changes in the atomic structure of a material resulting from the application of stressors (such as heat, nuclear radiation, oxygen/ozone, etc.). Furthermore, existing neutron scanning and detection systems necessarily utilize very high neutron fluxes (&gt;1E10 n/s-4.pi.) in order to minimize analysis time. Such systems can induce significant damage to both inorganic (such as metals) and organic materials. While neutron radiation primarily results in atomic displacement effects (which are highly detrimental to inorganics), it also induces a substantial degree of ionization within organic materials.
Based on the foregoing, it would be most desirable to provide an apparatus and method which allows an operator to more accurately assess the aging an in-situ degradable component in a substantially non-destructive manner and without requiring direct access to the component. Such apparatus and method could, for example, be used to estimate the aging of an electrical cable within a metallic conduit, or similarly to estimate the aging of a valve internal component while still installed within its host valve.